Yes, it is safe to have miso during pregnancy. As per a report published in the NHS Choices, pregnant women can eat certain products made of soy, and this list includes miso as well. Miso is a fermented product. Hence, it is rich in probiotic microorganisms.
According to a report published by the Harvard Medical School, many probiotics can prevent or control the symptoms of diarrhea in pregnant women, especially when it develops after taking antibiotics that destroy healthy intestinal organisms. Apart from that, miso is also rich in vitamin K, a nutrient essential for preventing a rare disorder known as vitamin K deficiency bleeding in babies.